Chemistry
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Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization
Chapters
Summary August 6th, 2018
We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.
Transcript
This method can help answer key questions about the ultra fast dynamics of polyatomic radical cations such as whether coherent vibrational motions are excited and how coherent exotation drives bond association. The main advantage of this technique is that polyatomic radical cations may be prepared in the ground electronic state, which significantly improves the ability to resolve coherent vibrational motions. Generally, individuals new to this method would struggle with setting up two independent beam path with identical path name in ensuring that the two beams propagate exactly parallelly or collinearly when necessary.
First, turn the femtosecond laser on and wait for about 30 minutes for it to stabilize. Position a 90/10 beam splitter after the laser output to generate two replica, which will be used to construct the pump and probe beam lines. Check the laser power before the beam splitter and the power of the transmitted beam to ensure that approximately 10%of the power is transmitted.
Direct the reflected beam into the optical parametric amplifier. Then, optimize the output power using the procedures in the manual. Next, set the optical parametric amplifier software to select the desired wavelength.
Direct the output beam from the optical parametric amplifier through the half wave plate and polarizer. Block the P polarized beam and direct the S polarized beam to the concave and convex mirrors to expand its diameter by a factor of five. Then, direct the expanded beam to the dichroic mirror.
To prepare the probe optical path, direct the beam that passes through the 90/10 beam splitter to the convex and concave mirrors to reduce its diameter by a factor of two. Direct the down collimated beam to a hollow retroreflector mounted on a motorized linear delay stage. Adjust the mounting knobs of the two flat mirrors prior to the retro reflector to ensure that the beam position after the retroreflector does not change when the stage is moved along its full travel range.
Next, insert a tunable neutral density filter after the delay stage to attenuate the power of the probe pulse, insert an iris after the neutral density filter to adjust the beam diameter, and direct the beam to the dichroic mirror. Now, insert a 15 millimeter diameter Beta-Barium Borate crystal after the dichroic mirror to double the wavelengths of both beams, thereby making them visible. Adjust the pump end probe beam alignments using the mirror mounts prior to the dichroic mirror such that the beams propagate collinearly through the TOFMS chamber and out from the other side.
Following this, place a fast photo dio detector a few centimeters in front of the window entrance to the TOFMS chamber in the path of the pump end probe beams. Attach the detector cable to a digital oscilloscope and independently locate the signals of the pump end probe pulses. Adjust the position of the motorized delay stage on the probe line such that the pump end probe signals in the oscilloscope are temporally overlapped.
If one signal is consistently in front of the other in the oscilloscope, move the mounts holding the motorized delay stage to shorten or lengthen the path length as needed. Then, remove the photodiode detector. Connect the desired sample to the TOFMS chamber and adjust the pressure.
Turn on the TOFMS power supply and check the voltages. Verify the operability of the data acquisition software with respect to the communication with both the motorized delay stage and the oscilloscope. Unblock the pump end probe beams and ensure that they are aligned into the TOFMS chamber.
Maximize the probe power by adjusting the neutral density filter. Set the pump power with the wave plate to a sufficiently high level to obtain satisfactory ion signal. Next, adjust the spacial position of the probe beam with the knobs on the dichroic mirror mount until either a spike in intensity of all ions is observed, or a significant depletion of the parent molecular ion and, or increase in fragment ion yields are observed.
Adjust the motorized delay stage position to produce a spike in total ion signal, which corresponds to zero time delay. Adjust the pump end probe pulse energies to obtain desired ion signals. In the data acquisition software, specify the scan length and step size.
Finally, run the data acquisition software to acquire the mass spectrum at each pump-probe delay. DMMP mass spectra taken at zero time delay and with only the pump pulse are shown here. There is little perceptible difference between the poorly overlapped and pump only spectra, which illustrates how to determine optimal spacial overlap of the pump end probe beams using the ion signals directly.
Mass spectral data obtained from one pump-probe scan with the flight time on the abscissa and pump-probe delay on the ordinate are displayed here. The raw data illustrates how changes in ion signals with the pump-probe delay in these experiments can be visualized without additional data workup. Time resolved DMMP ion signals from one pump-probe scan with optimized and poor spacial overlap of pump end probe beams are shown here.
These results illustrate the importance of optimizing the pump-probe spacial overlap to acquire high quality transient ion signals in the processed data. DMMP ion and the fragment PO2C2H4 ion transient ion signals taken using 800 nanometer and 1500 nanometer pump wave lengths are displayed here. The fast foray transform of the DMMP ion signals taken with 800 nanometer and 1500 nanometer pumps is shown here.
The peak at 750 wave numbers visible for the 1500 nanometer pump illustrates the frequency resolution under the scan settings used. While attempting this procedure, it's important to remember to double check the spacial develop of the pump and the probe beams before recording any experimental data. Following this procedure, ab initio or density functional theory calculations can be performed in order to determine which vibrational modes are coherently excited, as well as identify the electronic energy level or levels accessed by the probe-pulse exotation.
Don't forget that working with high powered lasers can be extremely hazardous and precautions such as wearing appropriate safety goggles should always be taken while performing this procedure.
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